Anisotropic Texture Filtering with Texture Data Prefetching

ABSTRACT

A circuit arrangement and method utilize texture data prefetching to prefetch texture data used by an anisotropic filtering algorithm. In particular, stride-based prefetching may be used to prefetch texture data for use in anisotropic filtering, where the value of the stride, or difference between successive accesses, is based upon a distance in a memory address space between sample points taken along the line of anisotropy used in an anisotropic filtering algorithm.

FIELD OF THE INVENTION

The invention is generally related to data processing, and in particularto processor architectures and execution units incorporated therein.

BACKGROUND OF THE INVENTION

As semiconductor technology continues to inch closer to practicallimitations in terms of increases in clock speed, architects areincreasingly focusing on parallelism in processor architectures toobtain performance improvements. At the chip level, multiple processorcores are often disposed on the same chip, functioning in much the samemanner as separate processor chips, or to some extent, as completelyseparate computers. In addition, even within cores, parallelism isemployed through the use of multiple execution units that arespecialized to handle certain types of operations. Pipelining is alsoemployed in many instances so that certain operations that may takemultiple clock cycles to perform are broken up into stages, enablingother operations to be started prior to completion of earlieroperations. Multithreading is also employed to enable multipleinstruction streams to be processed in parallel, enabling more overallwork to performed in any given clock cycle.

In addition, as processor architectures improve in terms of rawperformance, other considerations, such as the communication costs ofstoring and retrieving data, become significant factors in overallperformance. Data is typically organized within a memory address spacethat represents the addressable range of memory addresses that can beaccessed by a processor. Both the instructions forming a computerprogram and the data operated upon by those instructions are oftenstored in a memory system and retrieved as necessary by a processor whenexecuting the computer program. In order to balance cost, performance,and storage capacity, multi-level memory architectures have beendeveloped.

Often, a computer relies on a relatively large, slow and inexpensivemass storage system such as a hard disk drive or other external storagedevice, an intermediate main memory that uses dynamic random accessmemory devices (DRAM's) or other volatile memory storage devices, andone or more high speed, limited capacity cache memories, or caches,implemented with static random access memory devices (SRAM's) or thelike (e.g., L1, L2, L3, etc. caches). In some instances, instructionsand data are stored in separate instruction and data cache memories topermit instructions and data to be accessed in parallel. One or morememory controllers are then used to swap the information from segmentsof memory addresses, often known as “cache lines”, between the variousmemory levels to attempt to maximize the frequency that requested memoryaddresses are stored in the fastest cache memory accessible by themicroprocessor. Whenever a memory access request attempts to access amemory address that is not cached in a cache memory, a “cache miss”occurs. As a result of a cache miss, the cache line for a memory addresstypically must be retrieved from a relatively slow, lower level memory,often with a significant performance hit.

In some designs, prefetching is used to reduce the impact of cachemisses, and may be used both with instructions and the data processed byinstructions. From the standpoint of instructions, prefetching typicallyrefers to transferring instructions into a processor, or into a cache orother memory storage element disposed within or accessible by theprocessor, prior to an attempt to issue the instruction by theinstruction issue logic for the processor. Likewise, for non-instructiondata, prefetching typically refers to transferring data into aprocessor, or into a cache or other memory storage element disposedwithin or accessible by the processor, prior to a request for the databeing generated via execution of an instruction within the processor.

For program instructions, for example, instruction prefetching is oftenused to attempt to initiate the retrieval of instructions into aninstruction cache or buffer before the instructions are needed by aprocessor. Branch prediction may also be used, for example, to predictwhether certain conditional branches will be taken, with prefetchingused to initiate the retrieval of the instructions that are expected tofollow the conditional branches if those branches are or are not taken.

Prefetching may also be used for other types of data, and often takesadvantage of the fact that data requests are often somewhat regular innature for a particular set of program instructions. Stride-basedprefetching, for example, takes advantage of the fact that data is oftenretrieved in a pattern having a relatively constant offset, referred toas a “stride”, between successive accesses. Stride-based prefetchingtypically determines a difference between the memory addresses ofconsecutive accesses, and then prefetches additional data located one ormore multiples of that difference from the prior accesses.

Caching and prefetching often provide substantial performance gains indata-intensive algorithms that rely heavily on retrieved data. One areathat is particularly data-intensive is rasterization, and in particulartexture processing performed in a rasterization process utilized invarious image processing applications. Rasterization is a process in 3Dgraphics where three dimensional geometry that has been projected onto ascreen is “filled in” with pixels of the appropriate color andintensity. A texture mapping algorithm is typically incorporated into arasterization process to paint a texture onto geometric objects placedinto a scene.

In order to paint a texture onto a placed object in a scene, the pixelsin each primitive making up the object are typically transformed from 3Dscene or world coordinates (e.g., x, y and z) to 2D coordinates relativeto a procedural or bitmapped texture (e.g., u and v). The fundamentalelements in a texture are referred to as texels (or texture pixels), andbeing the fundamental element of a texture, each texel is associatedwith a single color. Due to differences in orientation and distance ofthe surfaces of placed geometric primitives relative to the viewer, apixel in an image buffer will rarely correspond to a single texel in atexture. As a result, texture filtering is typically performed todetermine a color to be assigned to a pixel based upon the colors ofmultiple texels in proximity to the texture mapped position of thepixel.

A number of texture filtering algorithms may be used to determine acolor for a pixel, including simple interpolation, bilinear filtering,trilinear filtering, and anisotropic filtering, among others. With manytexture filtering algorithms, weights are calculated for a number ofadjacent texels to a pixel, the weights are used to scale the colors ofthe adjacent texels, and a color for the pixel is assigned by summingthe scaled colors of the adjacent texels. The color is then eitherstored at the pixel location in a frame buffer, or used to update acolor that is already stored at the pixel location.

Bilinear filtering, for example, uses the coordinates of a texturesample to perform a weighted average of four adjacent pixels, weightedaccording to how close the sample coordinates are to the center of thepixel. Bilinear filtering often can reduce the blockiness of closerdetails, but often does little to reduce the noise that is often foundin distant details.

Trilinear filtering involves using MIP mapping, which uses a set ofprefiltered texture images that are scaled to successively lowerresolutions. The algorithm uses texture samples from the high resolutiontextures for portions of the geometry near to the camera, and lowresolution textures for the portions distant to the camera. MIP mappingoften reduces nearby pixelation and distant noise; however, detail inthe distance is often lost and needlessly blurred. The blurriness is dueto the texture samples being taken from a MIP level of the texture thathas been pre-scaled to a low resolution in both the x and y dimensionsuniformly, such that resolution is lost in the direction perpendicularto the direction that the texture is most compressed.

Anisotropic filtering involves taking multiple samples along a “line ofanisotropy” which runs in the direction that the texture is mostcompressed. Each of these samples may be bilinear or trilinear filtered,and the results are then averaged together. This algorithm allows thecompression to occur in only one direction. By doing so, less blurringoften occurs in more distant features.

However, it has been found that the performance of anisotropic filteringis greatly dependent upon the number of samples taken along the line ofanisotropy. Larger numbers of samples greatly improve image quality, butalso greatly increase the processing overhead of the algorithm. Inaddition, high quality anisotropic filtering often introducessubstantial memory bandwidth limitations due to the need to retrieve thetexture data necessary to perform the required calculations.

For example, assuming textures are stored in memory uncompressed andeach pixel uses 16 bytes of RGB color and alpha channel information, ata setting of 16 texel samples per line of anisotropy, the memory trafficfor each pixel of a rasterized polygon using anisotropic filtering wouldinvolve loading 256 bytes. Assuming, for example, a resolution of1024×768 pixels and animation running at 30 frames per second, thebandwidth needed for a full screen of anisotropic filtered textureswould be approximately 5.7 Gigabytes per second.

Given the high bandwidth required, it is crucial for performance reasonsto ensure that as much as possible of the required texture data iscached in a processor, as otherwise the time required to retrieve thetexture data into the cache would introduce substantial delays in ananisotropic filtering algorithm. However, particularly with largertextures and/or with long lines of anisotropy, even when texturecompression is used, the texture data will often span numerous cachelines, so the required texture data is often not cached when it isneeded by an anisotropic filtering algorithm, leading to unacceptableperformance degradation.

Therefore, a need exists in the art for a manner of improving theperformance of anisotropic filtering algorithms, particularly from thestandpoint of minimizing the performance penalties associated withhaving to retrieve texture data in association with such algorithms.

SUMMARY OF THE INVENTION

The invention addresses these and other problems associated with theprior art by incorporating texture data prefetching to prefetch texturedata used by an anisotropic filtering algorithm. In embodimentsconsistent with the invention, in particular, stride-based prefetchingis used, where the value of the stride, or difference between successiveaccesses, is based upon a distance in a memory address space betweensample points taken along the line of anisotropy used in an anisotropicfiltering algorithm.

Consistent with one aspect of the invention, a circuit arrangementincluding a memory storage element and processing logic coupled to thememory storage element and configured to perform anisotropic filteringusing a texture including texture data stored in a memory address space.The processing logic is configured to perform anisotropic filteringbased upon a plurality of sample points defined along a line ofanisotropy for the texture, and the processing logic is furtherconfigured to initiate a prefetch of texture data for a sample pointamong the plurality of sample points into the memory storage elementusing a stride value representative of a distance in the memory addressspace between sample points in the texture.

Consistent with another aspect of the invention, a method performsanisotropic texture filtering using a texture including texture datastored in a memory address space. The method includes determining aplurality of sample points along a line of anisotropy for the texture,determining a stride value representative of a distance in the memoryaddress space between sample points in the texture, initiating aprefetch of texture data for a sample point among the plurality ofsample points using the stride value, and performing anisotropicfiltering using the prefetched texture data.

These and other advantages and features, which characterize theinvention, are set forth in the claims annexed hereto and forming afurther part hereof. However, for a better understanding of theinvention, and of the advantages and objectives attained through itsuse, reference should be made to the Drawings, and to the accompanyingdescriptive matter, in which there is described exemplary embodiments ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of exemplary automated computing machineryincluding an exemplary computer useful in data processing consistentwith embodiments of the present invention.

FIG. 2 is a block diagram of an exemplary NOC implemented in thecomputer of FIG. 1.

FIG. 3 is a block diagram illustrating in greater detail an exemplaryimplementation of a node from the NOC of FIG. 2.

FIG. 4 is a block diagram illustrating an exemplary implementation of anIP block from the NOC of FIG. 2.

FIG. 5 is a block diagram of an exemplary texture oriented in a 3D imagespace, for which anisotropic filtering with texture data prefetchingconsistent with the invention may be performed.

FIG. 6 is a block diagram of a line of anisotropy for the texture ofFIG. 5.

FIG. 7 is a flowchart illustrating an exemplary implementation of aroutine for implementing anisotropic filtering with texture dataprefetching consistent with the invention.

FIG. 8 is a block diagram of one exemplary implementation of aprocessing unit suitable for implementing anisotropic filtering withtexture data prefetching consistent with the invention.

FIG. 9 is a block diagram of another exemplary implementation of aprocessing unit suitable for implementing anisotropic filtering withtexture data prefetching consistent with the invention.

DETAILED DESCRIPTION

Embodiments consistent with the invention implement texture dataprefetching into an anisotropic filtering algorithm to reduce the memoryperformance bottlenecks associated with retrieving texture data using inan anisotropic filtering algorithm. In embodiments consistent with theinvention, in particular, a form of stride-based prefetching is used,where the value of the stride, or difference between successiveaccesses, is based upon a distance in a memory address space betweensample points taken along the line of anisotropy used in an anisotropicfiltering algorithm.

In particular, it has been found that since the sample points along aline of anisotropy are evenly spaced along the line, and texture imagesare often stored in memory as evenly spaced sequential data, the lengthbetween each of the sample points in a memory address space oftencorresponds to a fixed data size interval. Since this interval often canbe calculated at the beginning of an anisotropic filtering algorithm foreach screen pixel, a caching architecture can be controlled so as toconcurrently work to ensure that the right texels are cached (i.e.,prefetching the texture data if it isn't already in the cache) while thetexels that have already been loaded into registers can be used in theweighted averaging portion of the algorithm. This allows the floatingpoint math of the algorithm and the texel fetching from memory to occurconcurrently, and thus decrease memory overhead and increaseperformance.

Hardware and Software Environment

Now turning to the drawings, wherein like numbers denote like partsthroughout the several views, FIG. 1 illustrates exemplary automatedcomputing machinery including an exemplary computer 10 useful in dataprocessing consistent with embodiments of the present invention.Computer 10 of FIG. 1 includes at least one computer processor 12 or‘CPU’ as well as random access memory 14 (‘RAM’), which is connectedthrough a high speed memory bus 16 and bus adapter 18 to processor 12and to other components of the computer 10.

Stored in RAM 14 is an application program 20, a module of user-levelcomputer program instructions for carrying out particular dataprocessing tasks such as, for example, word processing, spreadsheets,database operations, video gaming, stock market simulations, atomicquantum process simulations, or other user-level applications. Alsostored in RAM 14 is an operating system 22. Operating systems useful inconnection with embodiments of the invention include UNIX™, Linux™,Microsoft Windows XP™, AIX™, IBM's i5/OS™, and others as will occur tothose of skill in the art. Operating system 22 and application 20 in theexample of FIG. 1 are shown in RAM 14, but many components of suchsoftware typically are stored in non-volatile memory also, e.g., on adisk drive 24.

As will become more apparent below, embodiments consistent with theinvention may be implemented within Network On Chip (NOC) integratedcircuit devices, or chips, and as such, computer 10 is illustratedincluding two exemplary NOCs: a video adapter 26 and a coprocessor 28.NOC video adapter 26, which may alternatively be referred to as agraphics adapter, is an example of an I/O adapter specially designed forgraphic output to a display device 30 such as a display screen orcomputer monitor. NOC video adapter 26 is connected to processor 12through a high speed video bus 32, bus adapter 18, and the front sidebus 34, which is also a high speed bus. NOC Coprocessor 28 is connectedto processor 12 through bus adapter 18, and front side buses 34 and 36,which is also a high speed bus. The NOC coprocessor of FIG. 1 may beoptimized, for example, to accelerate particular data processing tasksat the behest of the main processor 12.

The exemplary NOC video adapter 26 and NOC coprocessor 28 of FIG. 1 eachinclude a NOC, including integrated processor (‘IP’) blocks, routers,memory communications controllers, and network interface controllers,the details of which will be discussed in greater detail below inconnection with FIGS. 2-3. The NOC video adapter and NOC coprocessor areeach optimized for programs that use parallel processing and alsorequire fast random access to shared memory. It will be appreciated byone of ordinary skill in the art having the benefit of the instantdisclosure, however, that the invention may be implemented in devicesand device architectures other than NOC devices and devicearchitectures. The invention is therefore not limited to implementationwithin an NOC device.

Computer 10 of FIG. 1 includes disk drive adapter 38 coupled through anexpansion bus 40 and bus adapter 18 to processor 12 and other componentsof the computer 10. Disk drive adapter 38 connects non-volatile datastorage to the computer 10 in the form of disk drive 24, and may beimplemented, for example, using Integrated Drive Electronics (‘IDE’)adapters, Small Computer System Interface (‘SCSI’) adapters, and othersas will occur to those of skill in the art. Non-volatile computer memoryalso may be implemented for as an optical disk drive, electricallyerasable programmable read-only memory (so-called ‘EEPROM’ or ‘Flash’memory), RAM drives, and so on, as will occur to those of skill in theart.

Computer 10 also includes one or more input/output (‘I/O’) adapters 42,which implement user-oriented input/output through, for example,software drivers and computer hardware for controlling output to displaydevices such as computer display screens, as well as user input fromuser input devices 44 such as keyboards and mice. In addition, computer10 includes a communications adapter 46 for data communications withother computers 48 and for data communications with a datacommunications network 50. Such data communications may be carried outserially through RS-232 connections, through external buses such as aUniversal Serial Bus (‘USB’), through data communications datacommunications networks such as IP data communications networks, and inother ways as will occur to those of skill in the art. Communicationsadapters implement the hardware level of data communications throughwhich one computer sends data communications to another computer,directly or through a data communications network. Examples ofcommunications adapters suitable for use in computer 10 include modemsfor wired dial-up communications, Ethernet (IEEE 802.3) adapters forwired data communications network communications, and 802.11 adaptersfor wireless data communications network communications.

For further explanation, FIG. 2 sets forth a functional block diagram ofan example NOC 102 according to embodiments of the present invention.The NOC in FIG. 2 is implemented on a ‘chip’ 100, that is, on anintegrated circuit. NOC 102 includes integrated processor (‘IP’) blocks104, routers 110, memory communications controllers 106, and networkinterface controllers 108 grouped into interconnected nodes. Each IPblock 104 is adapted to a router 110 through a memory communicationscontroller 106 and a network interface controller 108. Each memorycommunications controller controls communications between an IP blockand memory, and each network interface controller 108 controls inter-IPblock communications through routers 110.

In NOC 102, each IP block represents a reusable unit of synchronous orasynchronous logic design used as a building block for data processingwithin the NOC. The term ‘IP block’ is sometimes expanded as‘intellectual property block,’ effectively designating an IP block as adesign that is owned by a party, that is the intellectual property of aparty, to be licensed to other users or designers of semiconductorcircuits. In the scope of the present invention, however, there is norequirement that IP blocks be subject to any particular ownership, sothe term is always expanded in this specification as ‘integratedprocessor block.’ IP blocks, as specified here, are reusable units oflogic, cell, or chip layout design that may or may not be the subject ofintellectual property. IP blocks are logic cores that can be formed asASIC chip designs or FPGA logic designs.

One way to describe IP blocks by analogy is that IP blocks are for NOCdesign what a library is for computer programming or a discreteintegrated circuit component is for printed circuit board design. InNOCs consistent with embodiments of the present invention, IP blocks maybe implemented as generic gate netlists, as complete special purpose orgeneral purpose microprocessors, or in other ways as may occur to thoseof skill in the art. A netlist is a Boolean-algebra representation(gates, standard cells) of an IP block's logical-function, analogous toan assembly-code listing for a high-level program application. NOCs alsomay be implemented, for example, in synthesizable form, described in ahardware description language such as Verilog or VHDL. In addition tonetlist and synthesizable implementation, NOCs also may be delivered inlower-level, physical descriptions. Analog IP block elements such asSERDES, PLL, DAC, ADC, and so on, may be distributed in atransistor-layout format such as GDSII. Digital elements of IP blocksare sometimes offered in layout format as well. It will also beappreciated that IP blocks, as well as other logic circuitry implementedconsistent with the invention may be distributed in the form of computerdata files, e.g., logic definition program code, that define at variouslevels of detail the functionality and/or layout of the circuitarrangements implementing such logic. Thus, while the invention has andhereinafter will be described in the context of circuit arrangementsimplemented in fully functioning integrated circuit devices and dataprocessing systems utilizing such devices, those of ordinary skill inthe art having the benefit of the instant disclosure will appreciatethat circuit arrangements consistent with the invention are capable ofbeing distributed as program products in a variety of forms, and thatthe invention applies equally regardless of the particular type ofcomputer readable or signal bearing media being used to actually carryout the distribution. Examples of computer readable or signal bearingmedia include, but are not limited to, physical, recordable type mediasuch as volatile and non-volatile memory devices, floppy disks, harddisk drives, CD-ROMs, and DVDs (among others), and transmission typemedia such as digital and analog communication links.

Each IP block 104 in the example of FIG. 2 is adapted to a router 110through a memory communications controller 106. Each memorycommunication controller is an aggregation of synchronous andasynchronous logic circuitry adapted to provide data communicationsbetween an IP block and memory. Examples of such communications betweenIP blocks and memory include memory load instructions and memory storeinstructions. The memory communications controllers 106 are described inmore detail below with reference to FIG. 3. Each IP block 104 is alsoadapted to a router 110 through a network interface controller 108,which controls communications through routers 110 between IP blocks 104.Examples of communications between IP blocks include messages carryingdata and instructions for processing the data among IP blocks inparallel applications and in pipelined applications. The networkinterface controllers 108 are also described in more detail below withreference to FIG. 3.

Routers 110, and the corresponding links 118 therebetween, implement thenetwork operations of the NOC. The links 118 may be packet structuresimplemented on physical, parallel wire buses connecting all the routers.That is, each link may be implemented on a wire bus wide enough toaccommodate simultaneously an entire data switching packet, includingall header information and payload data. If a packet structure includes64 bytes, for example, including an eight byte header and 56 bytes ofpayload data, then the wire bus subtending each link is 64 bytes wide,512 wires. In addition, each link may be bi-directional, so that if thelink packet structure includes 64 bytes, the wire bus actually contains1024 wires between each router and each of its neighbors in the network.In such an implementation, a message could include more than one packet,but each packet would fit precisely onto the width of the wire bus. Inthe alternative, a link may be implemented on a wire bus that is onlywide enough to accommodate a portion of a packet, such that a packetwould be broken up into multiple beats, e.g., so that if a link isimplemented as 16 bytes in width, or 128 wires, a 64 byte packet couldbe broken into four beats. It will be appreciated that differentimplementations may used different bus widths based on practicalphysical limits as well as desired performance characteristics. If theconnection between the router and each section of wire bus is referredto as a port, then each router includes five ports, one for each of fourdirections of data transmission on the network and a fifth port foradapting the router to a particular IP block through a memorycommunications controller and a network interface controller.

Each memory communications controller 106 controls communicationsbetween an IP block and memory. Memory can include off-chip main RAM112, memory 114 connected directly to an IP block through a memorycommunications controller 106, on-chip memory enabled as an IP block116, and on-chip caches. In NOC 102, either of the on-chip memories 114,116, for example, may be implemented as on-chip cache memory. All theseforms of memory can be disposed in the same address space, physicaladdresses or virtual addresses, true even for the memory attacheddirectly to an IP block. Memory addressed messages therefore can beentirely bidirectional with respect to IP blocks, because such memorycan be addressed directly from any IP block anywhere on the network.Memory 116 on an IP block can be addressed from that IP block or fromany other IP block in the NOC. Memory 114 attached directly to a memorycommunication controller can be addressed by the IP block that isadapted to the network by that memory communication controller—and canalso be addressed from any other IP block anywhere in the NOC.

NOC 102 includes two memory management units (‘MMUs’) 120, 122,illustrating two alternative memory architectures for NOCs consistentwith embodiments of the present invention. MMU 120 is implemented withinan IP block, allowing a processor within the IP block to operate invirtual memory while allowing the entire remaining architecture of theNOC to operate in a physical memory address space. MMU 122 isimplemented off-chip, connected to the NOC through a data communicationsport 124. The port 124 includes the pins and other interconnectionsrequired to conduct signals between the NOC and the MMU, as well assufficient intelligence to convert message packets from the NOC packetformat to the bus format required by the external MMU 122. The externallocation of the MMU means that all processors in all IP blocks of theNOC can operate in virtual memory address space, with all conversions tophysical addresses of the off-chip memory handled by the off-chip MMU122.

In addition to the two memory architectures illustrated by use of theMMUs 120, 122, data communications port 126 illustrates a third memoryarchitecture useful in NOCs capable of being utilized in embodiments ofthe present invention. Port 126 provides a direct connection between anIP block 104 of the NOC 102 and off-chip memory 112. With no MMU in theprocessing path, this architecture provides utilization of a physicaladdress space by all the IP blocks of the NOC. In sharing the addressspace bi-directionally, all the IP blocks of the NOC can access memoryin the address space by memory-addressed messages, including loads andstores, directed through the IP block connected directly to the port126. The port 126 includes the pins and other interconnections requiredto conduct signals between the NOC and the off-chip memory 112, as wellas sufficient intelligence to convert message packets from the NOCpacket format to the bus format required by the off-chip memory 112.

In the example of FIG. 2, one of the IP blocks is designated a hostinterface processor 128. A host interface processor 128 provides aninterface between the NOC and a host computer 10 in which the NOC may beinstalled and also provides data processing services to the other IPblocks on the NOC, including, for example, receiving and dispatchingamong the IP blocks of the NOC data processing requests from the hostcomputer. A NOC may, for example, implement a video graphics adapter 26or a coprocessor 28 on a larger computer 10 as described above withreference to FIG. 1. In the example of FIG. 2, the host interfaceprocessor 128 is connected to the larger host computer through a datacommunications port 130. The port 130 includes the pins and otherinterconnections required to conduct signals between the NOC and thehost computer, as well as sufficient intelligence to convert messagepackets from the NOC to the bus format required by the host computer 10.In the example of the NOC coprocessor in the computer of FIG. 1, such aport would provide data communications format translation between thelink structure of the NOC coprocessor 28 and the protocol required forthe front side bus 36 between the NOC coprocessor 28 and the bus adapter18.

FIG. 3 next illustrates a functional block diagram illustrating ingreater detail the components implemented within an IP block 104, memorycommunications controller 106, network interface controller 108 androuter 110 in NOC 102, collectively illustrated at 132. IP block 104includes a computer processor 134 and I/O functionality 136. In thisexample, computer memory is represented by a segment of random accessmemory (‘RAM’) 138 in IP block 104. The memory, as described above withreference to FIG. 2, can occupy segments of a physical address spacewhose contents on each IP block are addressable and accessible from anyIP block in the NOC. The processors 134, I/O capabilities 136, andmemory 138 in each IP block effectively implement the IP blocks asgenerally programmable microcomputers. As explained above, however, inthe scope of the present invention, IP blocks generally representreusable units of synchronous or asynchronous logic used as buildingblocks for data processing within a NOC. Implementing IP blocks asgenerally programmable microcomputers, therefore, although a commonembodiment useful for purposes of explanation, is not a limitation ofthe present invention.

In NOC 102 of FIG. 3, each memory communications controller 106 includesa plurality of memory communications execution engines 140. Each memorycommunications execution engine 140 is enabled to execute memorycommunications instructions from an IP block 104, includingbidirectional memory communications instruction flow 141, 142, 144between the network and the IP block 104. The memory communicationsinstructions executed by the memory communications controller mayoriginate, not only from the IP block adapted to a router through aparticular memory communications controller, but also from any IP block104 anywhere in NOC 102. That is, any IP block in the NOC can generate amemory communications instruction and transmit that memorycommunications instruction through the routers of the NOC to anothermemory communications controller associated with another IP block forexecution of that memory communications instruction. Such memorycommunications instructions can include, for example, translationlookaside buffer control instructions, cache control instructions,barrier instructions, and memory load and store instructions.

Each memory communications execution engine 140 is enabled to execute acomplete memory communications instruction separately and in parallelwith other memory communications execution engines. The memorycommunications execution engines implement a scalable memory transactionprocessor optimized for concurrent throughput of memory communicationsinstructions. Memory communications controller 106 supports multiplememory communications execution engines 140 all of which runconcurrently for simultaneous execution of multiple memorycommunications instructions. A new memory communications instruction isallocated by the memory communications controller 106 to a memorycommunications engine 140 and memory communications execution engines140 can accept multiple response events simultaneously. In this example,all of the memory communications execution engines 140 are identical.Scaling the number of memory communications instructions that can behandled simultaneously by a memory communications controller 106,therefore, is implemented by scaling the number of memory communicationsexecution engines 140.

In NOC 102 of FIG. 3, each network interface controller 108 is enabledto convert communications instructions from command format to networkpacket format for transmission among the IP blocks 104 through routers110. The communications instructions may be formulated in command formatby the IP block 104 or by memory communications controller 106 andprovided to the network interface controller 108 in command format. Thecommand format may be a native format that conforms to architecturalregister files of IP block 104 and memory communications controller 106.The network packet format is typically the format required fortransmission through routers 110 of the network. Each such message iscomposed of one or more network packets. Examples of such communicationsinstructions that are converted from command format to packet format inthe network interface controller include memory load instructions andmemory store instructions between IP blocks and memory. Suchcommunications instructions may also include communications instructionsthat send messages among IP blocks carrying data and instructions forprocessing the data among IP blocks in parallel applications and inpipelined applications.

In NOC 102 of FIG. 3, each IP block is enabled to sendmemory-address-based communications to and from memory through the IPblock's memory communications controller and then also through itsnetwork interface controller to the network. A memory-address-basedcommunications is a memory access instruction, such as a loadinstruction or a store instruction, that is executed by a memorycommunication execution engine of a memory communications controller ofan IP block. Such memory-address-based communications typicallyoriginate in an IP block, formulated in command format, and handed offto a memory communications controller for execution.

Many memory-address-based communications are executed with messagetraffic, because any memory to be accessed may be located anywhere inthe physical memory address space, on-chip or off-chip, directlyattached to any memory communications controller in the NOC, orultimately accessed through any IP block of the NOC—regardless of whichIP block originated any particular memory-address-based communication.Thus, in NOC 102, all memory-address-based communications that areexecuted with message traffic are passed from the memory communicationscontroller to an associated network interface controller for conversionfrom command format to packet format and transmission through thenetwork in a message. In converting to packet format, the networkinterface controller also identifies a network address for the packet independence upon the memory address or addresses to be accessed by amemory-address-based communication. Memory address based messages areaddressed with memory addresses. Each memory address is mapped by thenetwork interface controllers to a network address, typically thenetwork location of a memory communications controller responsible forsome range of physical memory addresses. The network location of amemory communication controller 106 is naturally also the networklocation of that memory communication controller's associated router110, network interface controller 108, and IP block 104. The instructionconversion logic 150 within each network interface controller is capableof converting memory addresses to network addresses for purposes oftransmitting memory-address-based communications through routers of aNOC.

Upon receiving message traffic from routers 110 of the network, eachnetwork interface controller 108 inspects each packet for memoryinstructions. Each packet containing a memory instruction is handed tothe memory communications controller 106 associated with the receivingnetwork interface controller, which executes the memory instructionbefore sending the remaining payload of the packet to the IP block forfurther processing. In this way, memory contents are always prepared tosupport data processing by an IP block before the IP block beginsexecution of instructions from a message that depend upon particularmemory content.

In NOC 102 of FIG. 3, each IP block 104 is enabled to bypass its memorycommunications controller 106 and send inter-IP block, network-addressedcommunications 146 directly to the network through the IP block'snetwork interface controller 108. Network-addressed communications aremessages directed by a network address to another IP block. Suchmessages transmit working data in pipelined applications, multiple datafor single program processing among IP blocks in a SIMD application, andso on, as will occur to those of skill in the art. Such messages aredistinct from memory-address-based communications in that they arenetwork addressed from the start, by the originating IP block whichknows the network address to which the message is to be directed throughrouters of the NOC. Such network-addressed communications are passed bythe IP block through I/O functions 136 directly to the IP block'snetwork interface controller in command format, then converted to packetformat by the network interface controller and transmitted throughrouters of the NOC to another IP block. Such network-addressedcommunications 146 are bi-directional, potentially proceeding to andfrom each IP block of the NOC, depending on their use in any particularapplication. Each network interface controller, however, is enabled toboth send and receive such communications to and from an associatedrouter, and each network interface controller is enabled to both sendand receive such communications directly to and from an associated IPblock, bypassing an associated memory communications controller 106.

Each network interface controller 108 in the example of FIG. 3 is alsoenabled to implement virtual channels on the network, characterizingnetwork packets by type. Each network interface controller 108 includesvirtual channel implementation logic 148 that classifies eachcommunication instruction by type and records the type of instruction ina field of the network packet format before handing off the instructionin packet form to a router 110 for transmission on the NOC. Examples ofcommunication instruction types include inter-IP blocknetwork-address-based messages, request messages, responses to requestmessages, invalidate messages directed to caches; memory load and storemessages; and responses to memory load messages, etc.

Each router 110 in the example of FIG. 3 includes routing logic 152,virtual channel control logic 154, and virtual channel buffers 156. Therouting logic typically is implemented as a network of synchronous andasynchronous logic that implements a data communications protocol stackfor data communication in the network formed by the routers 110, links118, and bus wires among the routers. Routing logic 152 includes thefunctionality that readers of skill in the art might associate inoff-chip networks with routing tables, routing tables in at least someembodiments being considered too slow and cumbersome for use in a NOC.Routing logic implemented as a network of synchronous and asynchronouslogic can be configured to make routing decisions as fast as a singleclock cycle. The routing logic in this example routes packets byselecting a port for forwarding each packet received in a router. Eachpacket contains a network address to which the packet is to be routed.

In describing memory-address-based communications above, each memoryaddress was described as mapped by network interface controllers to anetwork address, a network location of a memory communicationscontroller. The network location of a memory communication controller106 is naturally also the network location of that memory communicationcontroller's associated router 110, network interface controller 108,and IP block 104. In inter-IP block, or network-address-basedcommunications, therefore, it is also typical for application-level dataprocessing to view network addresses as the location of an IP blockwithin the network formed by the routers, links, and bus wires of theNOC. FIG. 2 illustrates that one organization of such a network is amesh of rows and columns in which each network address can beimplemented, for example, as either a unique identifier for each set ofassociated router, IP block, memory communications controller, andnetwork interface controller of the mesh or x, y coordinates of eachsuch set in the mesh.

In NOC 102 of FIG. 3, each router 110 implements two or more virtualcommunications channels, where each virtual communications channel ischaracterized by a communication type. Communication instruction types,and therefore virtual channel types, include those mentioned above:inter-IP block network-address-based messages, request messages,responses to request messages, invalidate messages directed to caches;memory load and store messages; and responses to memory load messages,and so on. In support of virtual channels, each router 110 in theexample of FIG. 3 also includes virtual channel control logic 154 andvirtual channel buffers 156. The virtual channel control logic 154examines each received packet for its assigned communications type andplaces each packet in an outgoing virtual channel buffer for thatcommunications type for transmission through a port to a neighboringrouter on the NOC.

Each virtual channel buffer 156 has finite storage space. When manypackets are received in a short period of time, a virtual channel buffercan fill up—so that no more packets can be put in the buffer. In otherprotocols, packets arriving on a virtual channel whose buffer is fullwould be dropped. Each virtual channel buffer 156 in this example,however, is enabled with control signals of the bus wires to advisesurrounding routers through the virtual channel control logic to suspendtransmission in a virtual channel, that is, suspend transmission ofpackets of a particular communications type. When one virtual channel isso suspended, all other virtual channels are unaffected—and can continueto operate at full capacity. The control signals are wired all the wayback through each router to each router's associated network interfacecontroller 108. Each network interface controller is configured to, uponreceipt of such a signal, refuse to accept, from its associated memorycommunications controller 106 or from its associated IP block 104,communications instructions for the suspended virtual channel. In thisway, suspension of a virtual channel affects all the hardware thatimplements the virtual channel, all the way back up to the originatingIP blocks.

One effect of suspending packet transmissions in a virtual channel isthat no packets are ever dropped. When a router encounters a situationin which a packet might be dropped in some unreliable protocol such as,for example, the Internet Protocol, the routers in the example of FIG. 3may suspend by their virtual channel buffers 156 and their virtualchannel control logic 154 all transmissions of packets in a virtualchannel until buffer space is again available, eliminating any need todrop packets. The NOC of FIG. 3, therefore, may implement highlyreliable network communications protocols with an extremely thin layerof hardware.

The example NOC of FIG. 3 may also be configured to maintain cachecoherency between both on-chip and off-chip memory caches. Each NOC cansupport multiple caches each of which operates against the sameunderlying memory address space. For example, caches may be controlledby IP blocks, by memory communications controllers, or by cachecontrollers external to the NOC. Either of the on-chip memories 114, 116in the example of FIG. 2 may also be implemented as an on-chip cache,and, within the scope of the present invention, cache memory can beimplemented off-chip also.

Each router 110 illustrated in FIG. 3 includes five ports, four ports158A-D connected through bus wires 118 to other routers and a fifth port160 connecting each router to its associated IP block 104 through anetwork interface controller 108 and a memory communications controller106. As can be seen from the illustrations in FIGS. 2 and 3, the routers110 and the links 118 of the NOC 102 form a mesh network with verticaland horizontal links connecting vertical and horizontal ports in eachrouter. In the illustration of FIG. 3, for example, ports 158A, 158C and160 are termed vertical ports, and ports 158B and 158D are termedhorizontal ports.

FIG. 4 next illustrates in another manner one exemplary implementationof an IP block 104 consistent with the invention, implemented as aprocessing element partitioned into an instruction unit (IU) 162,execution unit (XU) 164 and auxiliary execution unit (AXU) 166. In theillustrated implementation, IU 162 includes a plurality of instructionbuffers 168 that receive instructions from an L1 instruction cache(iCACHE) 170. Each instruction buffer 168 is dedicated to one of aplurality, e.g., four, symmetric multithreaded (SMT) hardware threads.An effective-to-real translation unit (iERAT) 172 is coupled to iCACHE170, and is used to translate instruction fetch requests from aplurality of thread fetch sequencers 174 into real addresses forretrieval of instructions from lower order memory. Each thread fetchsequencer 174 is dedicated to a particular hardware thread, and is usedto ensure that instructions to be executed by the associated thread isfetched into the iCACHE for dispatch to the appropriate execution unit.As also shown in FIG. 4, instructions fetched into instruction buffer168 may also be monitored by branch prediction logic 176, which provideshints to each thread fetch sequencer 174 to minimize instruction cachemisses resulting from branches in executing threads.

IU 162 also includes a dependency/issue logic block 178 dedicated toeach hardware thread, and configured to resolve dependencies and controlthe issue of instructions from instruction buffer 168 to XU 164. Inaddition, in the illustrated embodiment, separate dependency/issue logic180 is provided in AXU 166, thus enabling separate instructions to beconcurrently issued by different threads to XU 164 and AXU 166. In analternative embodiment, logic 180 may be disposed in IU 162, or may beomitted in its entirety, such that logic 178 issues instructions to AXU166.

XU 164 is implemented as a fixed point execution unit, including a setof general purpose registers (GPR's) 182 coupled to fixed point logic184, branch logic 186 and load/store logic 188. Load/store logic 188 iscoupled to an L1 data cache (dCACHE) 190, with effective to realtranslation provided by dERAT logic 192. XU 164 may be configured toimplement practically any instruction set, e.g., all or a portion of a32b or 64b PowerPC instruction set.

AXU 166 operates as an auxiliary execution unit including dedicateddependency/issue logic 180 along with one or more execution blocks 194.AXU 166 may include any number of execution blocks, and may implementpractically any type of execution unit, e.g., a floating point unit, orone or more specialized execution units such as encryption/decryptionunits, coprocessors, vector processing units, graphics processing units,XML processing units, etc. In the illustrated embodiment, AXU 166includes a high speed auxiliary interface to XU 164, e.g., to supportdirect moves between AXU architected state and XU architected state.

Communication with IP block 104 may be managed in the manner discussedabove in connection with FIG. 2, via network interface controller 108coupled to NOC 102. Address-based communication, e.g., to access L2cache memory, may be provided, along with message-based communication.For example, each IP block 104 may include a dedicated in box and/or outbox in order to handle inter-node communications between IP blocks.

Embodiments of the present invention may be implemented within thehardware and software environment described above in connection withFIGS. 1-4. However, it will be appreciated by one of ordinary skill inthe art having the benefit of the instant disclosure that the inventionmay be implemented in a multitude of different environments, and thatother modifications may be made to the aforementioned hardware andsoftware embodiment without departing from the spirit and scope of theinvention. As such, the invention is not limited to the particularhardware and software environment disclosed herein.

Anisotropic Filtering With Texture Data Prefetching

To implement anisotropic filtering with texture data prefetching in amanner consistent with the invention, texture data is selectivelyprefetched based upon a stride value that is based on the line ofanisotropy for a texture placed in a scene. FIG. 5, for example,illustrates an exemplary scene 200 within which is displayed a texture202 oriented in a 3D environment. The scene is described using (x,y)pixel coordinates defined along x and y axes of an (x,y) coordinatesystem 204, with each pixel (x,y) representing the smallest addressableelement of a displayed image.

As shown in FIG. 6, texture 202 is described using (u,v) texelcoordinates defined along u and v axes of a (u,v) coordinate system 206.When placed in scene 200, e.g., on the surface of a graphical primitive,each texel of texture 202 may be mapped to a particular pixel in scene200 by transposing the (u,v) coordinate system of the texture onto the(x,y) coordinate system of the scene. The lines “u-v” and “x-y” in FIG.5 respectively show the relative orientation of the (u,v) coordinatesystem relative to that of the (x,y) coordinate system.

It will be appreciated that when performing rasterization on aprimitive, anisotropic filtering may be performed for each pixel in thescene or image that corresponds to the primitive. Each such pixel may bemapped to a particular texture sample point in the texture such that,for each pixel to be processed during rasterization, a correspondingtexture sample point may be calculated in (u,v) coordinates bytransposition of the (x,y) coordinate of the pixel to the (u,v)coordinate system. It will be appreciated that it is rare for an (x,y)coordinate for a screen pixel to map to integer values in the (u,v)coordinate system due to the variable scale, rotation and perspective ofthe surface of the primitive upon which the texture to be drawn, so thecorresponding (u,v) coordinates for a screen pixel are typically definedas floating point values. In the illustrated embodiment, an anisotropicfiltering algorithm is performed separately for each pixel correspondingto the primitive, so the anisotropic filtering algorithm typicallyreceives as its input either the (x,y) coordinates of the pixel to bedrawn (which are subsequently converted to (u,v) coordinates by thealgorithm), or the (u,v) coordinates of the texture sample pointcorresponding to that pixel.

FIG. 7, for example, illustrates one exemplary implementation of ananisotropic filtering routine 220 consistent with the invention, whichreceives at block 222 an input texture sample point defined in (u,v)coordinates.

Next, a line of anisotropy for the input texture sample point is definedalong a direction of maximum compression for the texture, which isdescribed with respect to the (u,v) coordinate system as follows:

au=max(du/dx,du/dy)

av=max(dv/dx,dv/dy)

where du/dx is the ratio between u texel coordinate units along the uaxis versus the screen (pixel) x coordinate along the x axis, and du/dyis the ratio between u texel coordinate units along the u axis versusthe screen (pixel) y coordinate along the y axis. Similarly, dv/dx isthe ratio between v texel coordinate units along the v axis versus thescreen (pixel) x coordinate along the x axis, and dv/dy is the ratiobetween v texel coordinate units along the v axis versus the screen(pixel) y coordinate along the y axis. The calculation of these valuesis performed in blocks 224 and 226 of FIG. 7.

An exemplary line of anisotropy 208 is illustrated in FIG. 6, centeredon an input texture sample point 210 with (u,v) coordinates, and havinglengths au, av along the u, v axes, respectively.

Returning to FIG. 7, after au and av are calculated for the line ofanisotropy, (u,v) coordinates for n sample points (which may also bereferred to as sub-sample points) are calculated in block 228. The nsample points are evenly spaced along the line of anisotropy, and the(u,v) coordinates of each such sample point i (where i=0 to n−1) may becalculated as follows:

ui=u+(i*au/(n−1)−au/2)

vi=v+(i*av/(n−1)−av/2)

To implement texture prefetching in association with anisotropicfiltering in a manner consistent with the invention, texture data forselected sample points among the n sample points is prefetched such thatretrieval of the texture data is accelerated to make the data availablefor use by the anisotropic filtering routine when needed. The texturedata may include, for example, color data from one or more texels in atexture map (including texture data from one or more texture maps, e.g.,as used in MIP-based filtering).

The prefetch of such texture data is based upon the locations of thesample points within the texture, and in particular, the locations ofthe sample points along the line of anisotropy calculated in the mannerdescribed above. It has been found, in particular, that since the samplepoints along the line of anisotropy are evenly spaced along the line,and since texture images are typically stored in memory as evenly spacedsequential data, the length between each of the sample points in amemory address space often corresponds to a fixed data size interval. Inaddition, since this interval often can be calculated at the beginningof an anisotropic filtering algorithm for each screen pixel, a cachingarchitecture can be controlled so as to concurrently work to ensure thatthe right texels are cached (i.e., prefetching the texture data if itisn't already in the cache) while the texels that have already beenloaded into registers can be used in the weighted averaging portion ofthe algorithm. This allows the floating point math of the algorithm andthe texel fetching from memory to occur concurrently, and thus decreasememory overhead and increase performance.

The prefetch of texture data generally results in the texture data beingretrieved into a higher level of a multi-level memory architecture sothat when an attempt is made to use the texture data, e.g., by loadingthe texture data into a working register in a processor, the texturedata is resident in a higher level of memory than it would otherwisehave been, thus reducing the latency associated with loading the texturedata into a working register. The prefetch therefore can generally beconsidered to result in the texture data being retrieved into a memorystorage element such as a cache (e.g., an L1 cache), or even a dedicatedbuffer or one or more registers in a register file.

The aforementioned interval between sample points, which is referred toherein as a prefetch stride value, is calculated based upon thelocations of sample points along the line of anisotropy. To do so,length values representative of the distance in texels between adjacentsample points along the u and v axes (designated herein as ulen andvlen) may be calculated in block 230 of FIG. 7 as follows:

ulen=au/(n−1)

vlen=av/(n−1)

In other embodiments, the interval between sample points need not becalculated between adjacent sample points. For example, the interval maybe based on the distance between two sample points that are separatedfrom one another by one or more other sample points.

The prefetch stride value, which represents the distance in the memoryaddress space between the adjacent sample points, is then calculated inblock 232 of FIG. 7 as follows:

stride=vlen*(Uw*D)+ulen*D

where Uw is the width of the texture map in pixels in the U directionand D is the data size in bytes (or other addressable unit) of eachpixel in the texture map. It will be appreciated that the data size maybe represented in units other than bytes. In addition, theaforementioned equation assumes that the texture map is stored in memorycontiguously and in a regular and uncompressed manner. In otherembodiments, the texture map may be stored in a compressed format,whereby the conversion between the distance in texels and the distancein the memory address space may be calculated as appropriate for theparticular manner in which the texture map is stored, which manner willbe apparent to one of ordinary skill in the art having the benefit ofthe instant disclosure.

Once the prefetch stride value is calculated, the value is used toinitiate prefetching of texture data for one or more sample points, asshown in block 234 of FIG. 7. By initiating the prefetching of thetexture data for the sample points, the texture data is moved to arelatively higher level in the memory hierarchy so that when the texturedata is needed by the anisotropic filtering routine, the latency ofretrieving that data is minimized. Desirably, the prefetch of texturedata for a sample point is initiated before an attempt is made to loadthe texture data into a working register during execution of theanisotropic filtering routine. Furthermore, in embodiments where eachsample point is separately processed (e.g., by separately applyingbilinear or trilinear filtering to each sample point to generate asample value for that sample point) prior to performing a weightedaverage of the sample points, in many instances, it is desirable toinitiate the prefetch of texture data for a sample point beforecompleting the calculations involving an earlier sample point tominimize the latency associated with processing the latter sample point.

Consequently, as shown in FIG. 7, once prefetching of the texture datafor one or more sample points has been initiated, routine 220 theninitiates a loop at block 236 to perform one or more texel loads foreach sample point, until such time as all sample points have beenprocessed. In addition, if it is desired to apply any filtering to eachsample point, then block 236 may also apply such filtering (e.g.,bilinear or trilinear filtering) for the sample point using the loadedtexture data. At that point, block 238 passes control to block 240 toperform a weighted average of the accumulated samples and generate anoutput texel value (typically a texel color) for the texel.

Prefetching texture data based upon a stride value may be implemented ina number of manners consistent with the invention, including any numberof conventional manners of prefetching data in general in a processor,whether implemented in hardware, in software, or in a combinationthereof.

For example, initiating a prefetch of texture data may be implementedprimarily in software, i.e., within the anisotropic filtering routine,such that the anisotropic filtering program code calculates the stridevalue and initiates the prefetch. In one embodiment, for example, theanisotropic filtering program code may issue cache hints to initiateretrieval of texture data for selected sample points. The cache hintsmay be implemented using dedicated instructions, or in the alternative,may be implemented using conventional load or touch instructions.

In another embodiment, initiating a prefetch of texture data may beimplemented at least partially in hardware, and may incorporate prefetchlogic within a processor to handle the prefetch of texture data. Theprefetch logic may be implemented, for example, using a stride-basedprefetcher, or using a sequencer that processes a dedicated instructionsupplied by the anisotropic filtering program code that specifies one ormore texture parameters to control how prefetching is to be performed atthe hardware level. In still another embodiment, prefetching may beperformed completely in hardware, and without any dedicatedfunctionality in software to manage or initiate the prefetching, e.g.,by tracking or monitoring memory access requests to determine theappropriate stride.

One exemplary hardware-based implementation is illustrated in greaterdetail in FIG. 8. In this figure, a processing unit 250 incorporating anexecution unit 252 (e.g., a fixed point execution unit) and a separatefloating point unit 254 coupled to a memory hierarchy 256 through anM-way L1 data cache 258, which in this implementation, functions as amemory storage element into which texture data may be prefetched. Thememory hierarchy 256 may include, for example, an L2, L3 or other lowerlevel cache, as well as volatile or non-volatile main storage, whetherlocal or remote relative to the processing unit. Processing unit 250 maybe implemented, for example, in an IP block such as an IP block 104 fromFIGS. 1-4. In the alternative, processing unit 250 may be implemented inother processor architectures that issue and execute instructions,including single or multi-core microprocessors or microcontrollers. Inaddition, other components in processing unit 250, e.g., register files,instruction units, etc. are not shown in FIG. 8 in order to simplify adiscussion of the invention.

Hardware-based prefetch logic 260 is used in processing unit 250 toperform stride-based prefetching in a manner consistent with theinvention. The prefetch logic is disposed above or before cache 258, aswell as before address translation logic 262 that converts effective orvirtual addresses (e.g., a base addresses generated by execution unit252 using an adder 264) into physical addresses, and thus operates oneffective or virtual addresses.

Prefetch logic 260 includes a stride register 266, within which isstored a stride value calculated in the manner described above. Amultiplexer 268 receives at one input the base address generated byexecution unit 252 and adder 264, and at another input the output of anadder 270 that is fed back to multiplexer 268. A state machine 272controls the operation of the prefetch logic. During normal memoryaccesses generated by execution unit 252, multiplexer 268 passes theoutput of adder 264 to one input of adder 270, while stride register 266is cleared such that the base address from adder 264 is passed unchangedto address translation logic 262.

When prefetching is initiated, a stride value is stored in strideregister 266, and state machine 272 asserts a prefetch value signal atthe select input of multiplexer 268 to feed back the prior base address(which corresponds to the location in the memory address space of thetexture data for the first sample point) to adder 270, such that theprior base address is summed with the stride value and passed to addresstranslation logic 262 to initiate a fetch of the texture data for thenext sample point into the L1 cache 258. On subsequent memory accesscycles, texture data for additional sample points may be prefetched bycontinuing to sum the stride value with the last address output fromadder 270. The number of cycles for which the prefetch logic will issuenew memory requests may be fixed (e.g., equal to the number of samplepoints in used for anisotropic filtering), or may be programmable.

In one embodiment, stride register 266 may be a special purpose registerthat is accessible by program code. In another embodiment, a dedicatedinstruction may be defined in an instruction set to control prefetching.A dedicated instruction may specify, for example, one or more textureparameters, including but not limited to one or more of the u, vfloating point values for the initial texture sample point, the startingaddress of the texture, the width in texels or bytes of each line of thetexture, the number of sample points to be used, the line of anisotropylengths (au, av), the pixel/texel coordinate ratios (du/dx, du/dy,dv/dx, dv/dy) or any other parameters that may be used in performinganisotropic filtering. By providing such texture parameters in aninstruction, the prefetch logic may perform some or all of theoperations associated with anisotropic filtering, as well as thestride-based prefetching. For example, as illustrated in FIG. 8, aseparate sequencer 274 may optionally be incorporated to process adedicated instruction and control the prefetch logic as appropriate forthe texture parameters provided in the instruction.

FIG. 9 illustrates another suitable hardware-based implementation of aprocessing unit 300, in which an execution unit 302 and floating pointunit 304 are coupled to a lower memory hierarchy 306 via an M-way L1data cache 308 functioning as the memory storage element into whichtexture data may be prefetched. For normal memory accesses, an adder 310generates a base address for execution unit 302, which is thentranslated from effective or real format to a physical address viaaddress translation logic 312. Rather than being disposed above the L1cache; however, prefetch logic 314 in processing unit 300 is disposedbelow or after L1 cache 308. Stride detection logic 316 monitors ortracks memory accesses made by L1 cache 308 (representing the cachemisses for the memory accesses made by execution unit 302) and attemptsto detect a stride value from such accesses. Stride calculation logic318 then calculates a suitable stride for use in stride-basedprefetching. The addresses output by L1 cache 308 to the lower levelmemory hierarchy 306 are fed in this implementation to one input of amultiplexer 320, the other input of which is fed back the output of anadder 322. Multiplexer 320 outputs to one input of adder 322, with theother input receiving a data signal from stride calculation logic 318.

For normal memory accesses, the memory addresses output by L1 cache 308are passed unchanged through multiplexer 320 and adder 322, as stridecalculation logic 318 does not output a stride value, and does notassert a stride detect value signal at the select input of multiplexer320. When a stride is detected during anisotropic filtering, however,the calculated stride value is output to adder 322 and the select signalof multiplexer 320 is asserted such that the prior memory address outputby L1 cache 308 is summed with the stride value and used to requestanother memory address separated by the stride value. Subsequent memoryaccess cycles may then request additional memory addresses correspondingto additional sample points for a fixed or programmable number of samplepoints. In this manner, stride-based prefetching may be performed by thehardware without specific functionality being implemented within theanisotropic filtering program code executed by the processing unit.

Various modifications may be made without departing from the spirit andscope of the invention. For example, rather than prefetching into acache, prefetching may be performed into a dedicated buffer, or into aregister or group of registers in a register file. Other modificationswill be apparent to one of ordinary skill in the art. Therefore, theinvention lies in the claims hereinafter appended.

1. A method of performing anisotropic texture filtering using a texturedefined in a texel coordinate system having u and v axes and a scenedefined in a pixel coordinate system having x and y axes, the methodcomprising, for an initial sample point in the texture at u,vcoordinates: determining a line of anisotropy, including determining au,av values for the line of anisotropy as follows:au=max(du/dx,du/dy)av=max(dv/dx,dv/dy) where du/dx is a ratio between texel coordinatesalong the u axis and pixel coordinates along the x axis, du/dy is aratio between texel coordinates along the u axis and pixel coordinatesalong the y axis, dv/dx is a ratio between texel coordinates along the vaxis and pixel coordinates along the x axis, and dv/dy is a ratiobetween texel coordinates along the v axis and pixel coordinates alongthe y axis; determining n sample points along the line of anisotropy,including determining ui, vi coordinates for each sample point i, wherei=0 to n−1, as follows:ui=u+(i*au/(n−1)−au/2)vi=v+(i*av/(n−1)−av/2) determining a length ulen between adjacent samplepoints along the u axis and a length vlen between adjacent sample pointsalong the v axis as follows:ulen=au/(n−1)vlen=av/(n−1) determining a stride value as follows:stride=vlen*(Uw*D)+ulen*D where Uw is a width of the texture in texelsalong the u axis and D is a data size for each texel in memory; for eachof the n sample points, loading texture data for such sample point andapplying a filter to the first sample point using the loaded texturedata for such sample point to generate a sample result; for each of then sample points after a first sample point, using the stride value toinitiate a prefetch of the texture data for such sample point prior toattempting to load the texture data for such sample point; and averagingthe sample results for the n sample points.
 2. A circuit arrangement,comprising: a memory storage element; and processing logic coupled tothe memory storage element and configured to perform anisotropicfiltering using a texture including texture data stored in a memoryaddress space, the processing logic configured to perform anisotropicfiltering based upon a plurality of sample points defined along a lineof anisotropy for the texture, and the processing logic furtherconfigured to initiate a prefetch of texture data for a sample pointamong the plurality of sample points into the memory storage elementusing a stride value representative of a distance in the memory addressspace between sample points in the texture.
 3. The circuit arrangementof claim 2, wherein the memory storage element comprises at least one ofa register and a buffer.
 4. The circuit arrangement of claim 2, whereinthe memory storage element comprises a cache.
 5. The circuit arrangementof claim 4, wherein the processing logic comprises prefetch logiccoupled to the cache and configured to initiate the prefetch of texturedata into the cache.
 6. The circuit arrangement of claim 5, wherein theprefetch logic includes stride detection logic configured to detect thestride value by tracking memory accesses.
 7. The circuit arrangement ofclaim 5, wherein the prefetch logic is responsive to a dedicatedinstruction including at least one texture parameter, and wherein theprefetch logic is configured to calculate the stride value based uponthe at least one texture parameter.
 8. The circuit arrangement of claim5, wherein the prefetch logic includes a stride register configured tobe loaded with the stride value responsive to program code executed bythe processing logic.
 9. The circuit arrangement of claim 4, wherein theprocessing logic is configured to initiate the prefetch of texture datainto the cache responsive to program code executed by the processinglogic.
 10. The circuit arrangement of claim 2, wherein the processinglogic is configured to load the prefetched texture data into a workingregister in response to anisotropic filtering program code executed bythe processing logic, and wherein the processing logic is configured toinitiate the prefetch of the texture data before an attempt is made toload the texture data into the working register by the anisotropicfiltering program code.
 11. The circuit arrangement of claim 10, whereinthe processing logic is configured to calculate a sample value for thesample point by applying a filter to the prefetched texture data inresponse to the anisotropic filtering program code.
 12. The circuitarrangement of claim 11, wherein the processing logic is configured toapply at least one of a bilinear filter and a trilinear filter to theprefetched texture data.
 13. The circuit arrangement of claim 11,wherein the sample point is a first sample point, and wherein theprocessing logic is configured to initiate a prefetch of texture datafor a second sample point among the plurality of sample points beforecompleting the calculation of the sample value for the first samplepoint.
 14. The circuit arrangement of claim 11, wherein the processinglogic is configured to calculate a sample value for each of theplurality of sample points, and average the sample values for theplurality of sample points.
 15. An integrated circuit device includingthe circuit arrangement of claim
 2. 16. A program product comprising acomputer readable medium and logic definition program code resident onthe computer readable medium and defining the circuit arrangement ofclaim
 2. 17. A method of performing anisotropic texture filtering usinga texture including texture data stored in a memory address space, themethod comprising: determining a plurality of sample points along a lineof anisotropy for the texture; determining a stride value representativeof a distance in the memory address space between sample points in thetexture; initiating a prefetch of texture data for a sample point amongthe plurality of sample points using the stride value; and performinganisotropic filtering using the prefetched texture data.
 18. The methodof claim 17, wherein determining the stride value includes determining adistance in the memory address space between adjacent sample points inthe texture.
 19. The method of claim 17, wherein initiating the prefetchof texture data includes initiating the prefetch of the texture datainto a memory storage element selected from the group consisting of aregister, a buffer, and a cache.
 20. The method of claim 17, whereindetermining the stride value is performed by prefetch logic configuredto track memory accesses and determine the stride value based upon thetracked memory accesses.
 21. The method of claim 17, wherein determiningthe stride value is performed by prefetch logic responsive to adedicated instruction including at least one texture parameter, andwherein the prefetch logic is configured to calculate the stride valuebased upon the at least one texture parameter.
 22. The method of claim17, wherein determining the stride value is performed by prefetch logicincluding a stride register configured to be loaded with the stridevalue.
 23. The method of claim 17, further comprising attempting to loadthe prefetched texture data into a working register in response toanisotropic filtering program code, wherein initiating the prefetch ofthe texture data is performed before an attempt is made to load thetexture data into the working register by the anisotropic filteringprogram code.
 24. The method of claim 23, wherein the sample point is afirst sample point, the method further comprising: calculating a samplevalue for the first sample point by applying a filter to the prefetchedtexture data in response to the anisotropic filtering program code; andinitiating a prefetch of texture data for a second sample point amongthe plurality of sample points before completing the calculation of thesample value for the first sample point.
 25. The method of claim 24,further comprising: calculating a sample value for each of the pluralityof sample points; and averaging the sample values for the plurality ofsample points.